Thickness reduction control system



`lume 10, 1969 c. H. COLEMAN ETAL THICKNESS REDUCTION CONTROL SYSTEM Filed Feb. 1. 1967 ,w/ @www DW mear `Arrow/m9 United States Patent O 3,448,600 THICKNESS REDUCTION CONTROL SYSTEM Charles H. Coleman, Groton, and James H. Torrance,

Uncasville, Conn., assgnors to General Dynamics Corporation, New York, N.Y., a corporation of Delaware kFiled Feb. 1, 1967, Ser. No. 613,293 Int. Cl. B21b 3.7/12, 31/08, 13/00 U.S. Cl. 72-8 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to systems for controlling installations which reduce the thickness of material passing therethrough. The invention is applicable to various sorts of such installations (as, say, bar mills, tube mills, and die drawing installations) which are remployed to effect a reduction in thickness in one or more dimensions of lengths of materials of various compositions and shapes. For the purpose, however, of making clear the character of the invention, it is disclosed herein as being specifically embodied in a control system for a multi-stand rolling mill for metal sheet or strip.

In such a mill, the -strip is reduced in thickness by the work rolls of each stand in accordance with the size of the gap between those rolls. Because variations in thickness of the input strip to the rolls tends to a variable forcing apart of the rolls and because of other factors, it is necessary to adjust the relative spacing between the rolls (i.e., adjust the size of the roll gap) in orderto avoid undesirably large variations in thickness of the output strip from the rolls. To that end, numerous closed loop control systems have been proposed in which one or more signals of a condition of the moving strip (e.g., the instantaneous input thickness of the strip) are utilized to provide an output signal indicative of the instantaneous relative position the rolls should have to minimize the variation in thickness of the output strip from a desired thickness. Such output signal is then used to actuate r-oll adjusting means (e.g., a screw-down motor) to adjust the rolls to assume the relative position indicated by that signal. Such proposed position-determining systems usually have, however, the disadvantage, that they have too slow a response to compensate adequately for rapid variations in the input thickness of the strip.

U.S. Patent 3,197,986 toFreedman et al. discloses how to overcome such disadvantage by replacing the usual position-computing closed loop system by a closed loop control system which (a) computes the force the rolls should exert on the strip to obtain a desired output thickness, and (b) applies to the rolls an actual kforce corre- `sponding to such computed force by fa fast acting force generating device such as a wedge actuator. Wedge actuators of such sort are disclosed for example, in the Freedman et al. patent and in copending application Ser. No.

ice

i 405,749 tiled Oct. 22, 1964, now Patent No. 3,355,925,

and owned by the assignee hereof.

In the system of the Freedman et al. patent, the input thickness signal for the force-computing closed loop is derived from a conventional thickness gauge on the input side of the stand controlled by the system. Such thickness signal does not, however, reflect in its value various other gross changes in the condition of the moving strip as, for example, the chemistry, hardness, and temperature of the material of the strip and the width of the strip. In addition the thickness signal inherently contains low frequency noise due to internal and external sources (e.g., random nature of the gauge and fluids on the strip).

In U.S. Patent 3,111,046 to Koss et al. for a rolling mill, it is proposed to employ a signal derived from a load cell for the first stand as an error signal for a closed loop system controlling the thickness reduction effected by the rolls of the second stand. Because, however, the Koss et al. closed loop system is a position-computing system rather than a force-computing system of the sort described, the Koss et al. system is too slow to respond adequately to rapid fluctuations in the mentioned error signal. Hence, in the Koss et al. system, a rapid variation in the input thickness of strip to the second stand will cause a corresponding variation in thickness to appear in the output strip from that stand.

It is accordingly an object of this invention to control the thickness reducing action on material of the working means at least one stand of a multi-stand thicknessreducing installation by controlling the adjustment of such working means by force-generator means responsive to a signal of the force from such material on the working means of a preceding stand, said force-generator means being responsive to rapid fluctuations in such signal t0 correspondingly control said action.

Another object of this invention is to control the thickness reducing action on material of a thickness-reducing installation by means which compares a force signal derived from the thickness reducing action with a reference level for that signal, and which means then utilizes the result of the comparison to compute and apply to the working means a force which should yield a desired thickness reduction.

Still another object of thisinvention is to establish the mentioned reference level by passage of an initial portion of such material through the mentioned working means.

These and other objects are realized in accordance with the invention by providing for a multistand installation a control system wherein force-computation means responds to separate signals of the force from the material on the working means of one stand and of the force from the material on the working means of a preceding stand to derived an output representative of a computed force value which should yield a desired thickness of the strip emerging from the one stand. That output is then utilized to actuate a fast acting force generator means which applies to the working means of the one stand a force in accordance with said computed force value. Both the force computation means and the force-generator means have a fast enough response to follow accurately any rapid variations in the value of the mentioned force signals.

As another aspect of the invention, the force signal from the one stand is compared -with a reference level established for that signal to derive from such comparison an error signal representative of the difference between such signal and level. That error signal then controls the force-computation means to actuate the force-generator means as a function of that error signal.

As still another aspect of the invention, a reference level of such sort is established for either one or both of the force signal from the one stand and the force signal from the said preceding stand by passage of an initial portion of the material through those stands.

For a better understanding of the invention, reference is made to the following description of an exemplary embodiment thereof and to the accompanying figure which is a schematic diagram of a portion of a six stand strip rolling mill and of a control system associated with that portion.

In the description which follows, elements which are counterparts of each other are identified as such by using the same reference numeral for each element, but are distinguished from each other by using a prime sufiix for the reference numeral of one of the counterpart elements. It is to be understood that, unless the context otherwise requires, a description of any one element is to be taken as equally applicable to a counterpart thereof.

Referring now to the figure, the reference numeral designates a hot strip or sheet of iron or steel being reduced in thickness by being passed through the fourth stand 11 and the fifth stand 12 of a six stand rolling mill which may be similar to that disclosed in copending U.S. application 564,817 filed July 13, 1966, now abandoned and owned by the assignee hereof. Since the control system to be discussed is associated only with the fourth and fifth stands, the remaining stands of the six stand mill are not shown in the ligure.

Stand 11 is a conventional stand comprised of a frame 15 and -working means in the form of lower and upper work rolls 16 and 17 between which the strip 10 passes. The Work rolls 16 and 17 are dirven by a motor 20' which may be controlled as disclosed in U.S. application Ser. No. 564,817. The lower work roll 16 is contacted by a lower back-up roll 18 While the upper work roll 17 is contacted by a back-up roll 21, both disposed in roll chocks 19 and 22 supported within frame 15 to be vertically slidable therein. Roll chock 22 is subjected to pressure from a screw-down mechanism including a screwdown motor 23 and a screw-down control unit 24. A load sensing means in the form of a load cell 25 is disposed above roll chock 22 between the chock and frame 15. Load cell 25 produces on a lead 26 a sensed force Fa signal, i.e., an electrical signal which varies in amplitude in accordance with the force exerted by the strip 10 on the Work rolls 16 and 17.

The fifth stand 12 is like stand 11 in that stand 12 is comprised of frame 15', lower and upper work rolls 16 and 17', and driven by motor 20', a lower back-up roll 18 in chock 19', and upper back-up roll 21' in vertically slidable chock 22', a screw-down -motor 23 controlled by screw-down control unit 24 for exerting adjustable pressure on chock 22', and a load-cell 25 productive on lead 26' of an electrical Fa signal representative in amplitude of the force exerted on rolls 16' and 17 from the strip 10. As distinct from stand 11, however, stand 12 also includes a fast-acting force-generator means 30 interposed between chock 22' and the screw-down mechanism for that chock. Device 30 is preferably a wedge actuator such as is disclosed in U.S. Patent 3,197,986 (Freedman et al.) or in copending application Ser. No. 405,749 filed Oct. 22, 1964 and owned by the assignee hereof.

In the operation of the shown installation, the work rolls 16 and 17 of stand 11 reduce the thickness of strip 10, and a further reduction in strip thickness is effected by the work rolls 16 and 17 of stand 12. In stand 111, the thickness of the emerging strip depends upon the load on the stand and the initial position setting of the screw-down mechanism for that stand. Over the long term, the load on stand 12 is determined by the positional 4 setting of the screw-down mechanism for the latter stand. The short term loading on stand 12 and the output thickness of the strip from that stand are, however, primarily determined by a control system of which the device 30 forms a part, and which control system will now be described in detail.

While the sensed force or Fa signal from load cell 25 is representative of the force exerted from the strip on the work rolls 16 and 17 of stand 11, that signal is also representative of the output thickness of the strip emerging from that stand. Hence, such Fa signal is also representative of the input thickness of strip 10` in relation to stand 12.

Conventionally, the input thickness signal for stand 12 would be derived from an X-ray thickness gauge and should be corrected for gross changes in one or more of the parameters of the rolling operation such as the chemistry (i.e., composition) of the strip and its hardness and temperature. The Fa signal from load cell 25 is, however inherently sensitive to the changes in the aforementioned variables, and, accordingly, is superior to a signal derived from an X-ray gauge as an input thickness signal for stand 12.

Moreover, load cell 25 has a very fast response to changes in the force exerted from strip 10 on work rolls 16 and 17. Accordingly, the Fa signal from load cell 25 is adapted to manifest rapid changes in thickness of the input strip to stand 12 as rapid fluctuations in the amplitude of the load cell signal Fa.

The Fa signal from load cell 25 is fed via lead 26 to the summing amplifier 50. Another input t0 amplifier 50 is a signal Fr from lead 46 which is opposite in polarity to Fa. The Fr signal is obtained from the lock-0n device 45 and works as follows. As the leading edge of the strip 10 is sensed by strip detector 37, which may be, for example, a photocell, the blade 34 of a switching means contacts a contact 40 and connects one output of amplifier 50 to the lock-on device 45. This signal is integrated until a small predetermined error exists at the output of amplifier 50, at which time the blade 34 is switched to contact 47 which is at ground reference, and the integrated signal Fr is held at the output of unit 45 at that point until the tail end of the strip leaves the stand.

Because the input signals Fa and Fr to amplifier 50 are of opposite polarity, the amplifier provides as an output on lead 51 a signal AF which is representative in magnitude and polarity of the deviation of the Fa signal from the F, signal. Hence, the Fr signal acts as a reference signal for the 1:"a signal. While the Fr signal is indicative of a reference level for the force from strip 10 on the work rolls of stand 11, the Fr signal is also representative of a reference level for the thickness of the input strip to stand 12 in the same way as the Fa signal is representative of the instantaneous variations in thickness of such input strip. Note, moreover, that the Fr signal represents a thickness reference level which is automatically set by the leading portion of strip 10iy itself rather than a reference thickness level set in accordance with the judgment of the operator of the mill. Hence, the AF signal is representative of the instantaneous deviation in thickness of the input strip to stand 12 from a reference thickness level set for that `particular strip by the measured average thickness of the leading portion of such strip.

The analog AF signal on lead 51 is converted by analogto-digital converter 60 to digital form and is then passed through a delay device `61 which `may be, say, a digital shift register. Device 61 is controlled by shift pulses from a tachometer 62 driven by work roll 17 so as to cause the AF signal to be'delayed in device 61 for a time just equal to that required for the strip portion whose thickness is then represented by the AF signal to pass from the bite between the Work rolls of stand 11 to the bite between the work rolls of stand 12. However, due to the fact the back-up rolls 18 and 21 may be eccen- '5 tric and cause fiuctuation 180 degrees out of phase with the AF signal, the delay may be adjusted so that the delay is advanced one half of the circumference of the back-up roll, thus putting this signal in phase with actual thickness variation and the roll gap of stand 12. Hence, the AF signal at the output of device 61 represents at any instant the thickness value of the portion of strip which is just entering the bite between the Work rolls of stand 12. To make that output AF signal more convenient to operate on, it is reconverted by a digital-to-analog converter 63 into analog form. From converter 63, the analog AF signal is fed by lead 64 as a correction signal to a Iclosed loop system Iwhich provides the primary thickness control for stand 12, and which is as follows.

Load cell 25' of stand 12 generates on lead 26', a sensed force signal F a representative of the force exerted from strip 110 on the work roll 16', |17' of that stand. The Fa signal is led to the input of amplifier 90 where another input signal from lead 46' KF'r is opposite in polarity to F'a. The KF'r signal is obtained from modifying the F',t signal by the temperature of the strip entering stand 12 sensed by temperature gauge 91. This process is accomplished in block 70 ywhere the F'r signal is weighted as a function of the temperature of the strip. The second lock-on device works the same as the first except for the fact that temperature is also included. Amplifier 90 sums the F'a and KF'r signals of opposite polarity, as a differential amplifier which produces on its output lead l91 a AF' signal representative of the magnitude and polarity of the deviation in amplitude of the F', signal from a reference force level represented by the KF', signal and established by the leading portion of strip 10. Because the KF'r component of the AF' signal is provided as described by lock-on unit 45', the force deviation represented by the AF' signal is a deviation from a reference level of force which is automatically set by the strip itself rather than being set in accordance with the judgment of the operator of the mill.

The AF signal on lead 91 is supplied to the input of a summing amplifier 95 providing an output error signal Fc. During the period when lock-on unit 45' is building up signal F 'r, error signal Fc is disconnected. When blade 40' disconnects AF yfrom unit 45, a second blade 81 connects to point 82, thereby coupling Fc by lead 96 to forcegenerator device 30 to control the force exerted by that device on chock 22' and load cell 25'. Elements 25', 90, 95, and 30 thus form a closed loop servo system providing the main thickness control loop for stand 12.

If that control loop were to be energized only by the signals F', and KFT, the loop would operate to actnate device 30 to equalize the force F', exerted thereby to the reference force level FK'r. That is, the operation of the loop would be such that if constant thickness but varying temp strip were entering the stand the system would produce constant thickness strip leaving the stand.

However, due to the fact that the entering strip will not only vary in temperature, but thickness, the system makes use of the AF signal to command the force exerted by device 30 to reduce the thickness of the strip passing through work rolls 16' and 17 to a desired output thickness value.

As stated, the AF signal on lead 64 represents the instantaneous deviation in thickness of the input strip to stand 12 from a nominal or reference thickness set for that whole strip by the leading portion of said strip. That AF signal is passed through a signal-mixing network 100 where it is modified to become a KAF signal by exteriorly introduced constant correction signals K1, K2, K3 which may, for example, be representative of, respectively, (a) the type of material of which strip is formed, (b) a nominal thickness for the strip between the stands 11 and 12, and (c) the width of the strip material. The KAF signal is then supplied via lead 101 (and with a polarity opposite to that of the AF signal) to an input of summing amplifier yseparate from the amplifier input for the AF' signal.

For the purpose of using the tension of the strip as a factor in the computation of the control signal Fc, loopers and 106 develop respectively corresponding signals t1 and t2 which lare representative of, respectively, the tension of the strip between stands 11 and 12 and the tension of the strip on the output side of stand 12. The t1 and t2 signals are passed through respective signal weighting networks 107, 108 (e.g., potentiometers) and are then combined in a `summing amplifier 109 to produce on an output lead 110 for that amplifier an instantaneous tension signal t( If constant tension loopers are employed, this signal may be omitted from the control.

The thickness of strip 10 on the output side of the last stand (not shown) is monitored by a follow-on thickness gauge 115, for example, an X-ray gauge, developing a AG2 sign-al which is representative of the deviation, if any, of the output thickness of the strip from a desired output thickness therefor. High frequency variations in the AG2 signal are removed by a low pass filter 116. Hence, on the output lead 117 from the filter, the AG2 signal is representative only of long term variations in the output thickness of the strip. As disclosed in application Ser. No. 565,526 filed July l5, 1966 and owned by the assignee hereof, such AGZ signal from the follow-on gauge is adapted to correct for steady state errors produced by the control system because of drift in the gain of the surnming amplifiers or similar long term changes in the other operating parameters of the system.

Finally, the described control system provides for the introduction thereinto on a lead 120 of a manually set M signal adapted to be set to a value which will compensate for any residual inaccuracy in the operation of the control system.

The t signal on lead 110, the AG2 signal on lead 117 and M signal on lead 120 are supplied to separate inputs of la summing amplifier to provide on an output lead 126 for that amplifier an S signal representative of the combined inputs to that amplifier. The S signal is supplied to amplifier 95 as an input in addition to the AF and KAF signals applied thereto. By virtue of its summing action, that amplifier renders the Fc error signal on lead 96 a function of each of the input signals supplied to that amplifier.

Thus, the shown system computes the force to be applied by device 30 as a function of all the signals which have been described. The computed force value is the force value represented by the error signal Fc, and is the whole force value calculated as being required to be exerted by force generator 30 in order to minimize variations in thickness in the strip emerging from stand 12. Roughly speaking:

where the term C, represents the correction effect in the closed loop of the ith one of the desecribed correction signals.

The overall operation for one run of the shown installation is as follows. The strip 10 to be reduced in thickness during that run is advanced through the stands of the installation. When the front end of the strip passes strip detector 37, the resulting signal actuates the first lock-on unit 45. That unit develops from the Fa signal a held reference Fr signal representative of the average force in the stand for the leading portion of the strip. Once the lock-on is complete, the output of summing amplifier 50 produces a AF signal representative of the deviation of the Fa signal from the Fr signal. After being delayed by unit 61 for the time required for the strip portion whose thickness deviation is represented by AF to move from the bite between the work rolls of stand 11 to 7 the bite between the work rolls of stand 12, the AF signal is applied to the force-computing closed-loop thickness control system of stand 12 as one of the major correction signals for that system.

In the closed loop system for stand 12, the signal from strip detector 37 causes the second lock-on unit 45 to activate. The second lock-on unit 45' develops from the F'g, signal a reference signal KF'r representative of the average force in the stand for the leading portion of the strip modified by the instantaneous temperature changes in the strip. At the end of the second lock-on, summing amplifier 90 produces the AF force signal for closed loop computation.

Also included in that loop is summing amplifier 95 by which the correction signal AF is injected into the loop after being weighted by the factor K determined by three trimming or correction signals K1, K2 and K3. Other correction signals directly or indirectly introduced into the loop are the previously described signals T, t1, t2, G2 and M.

In response to all the mentioned signals, the control system computes a force error signal Fc which controls directly the amount of force exerted by force generator 30 on elements 22 and 25', and, indirectly, the thickness reduction instantaneously effected on Strip by the work rolls 16 and 17 of stand 12.

The above-described embodiments being exemplary only, it is to be understood that additions thereto, modifications thereof and omissions therefrom can be made without departing from the spirit of the invention and, that the invention comprehends embodiments differing in form and/ or detail from that embodiment which has been specifically described. For example, the invention is applicable to installations having more or less than six stands and to installations wherein the control system is associated with stands other than the two stands which have been specifically described. Accordingly, the invention is not to be considered as limited save as is consonant with the recitals of the following claims.

We claim:

1. In a mill control system in which material moves through successive stands and passes in each stand through adjustable working means to be reduced in thickness in accordance with the adjustment of such means, and in which force-computation means responds to a sensed force signal representative of force from said material on the working means of one of said stands to control forcegenerator means to apply to such working means a force which adjusts such means and is of a value computed as being that required to produce a desired thickness reducing action on said material by said working means, the improvement comprising, load-sensing means coupled to the working means of a stand preceding said one stand and responsive to force from said material on said latter working means to develop an additional signal representative of said last-named force, and means to supply a correction signal which is a function of said additional signal as an input to said force-computation means so as to render the control of said force-generator means a function of both said sensed force signal and said correction signal.

2. The improvement as in claim 1 further comprising, time-integrator lock-on means responsive to an input signal thereto to provide a held output which is a function of such signal as integrated over a time period during which said material is first passing through said one stand, signal-combining means responsive to two respective signal inputs thereto to supply to said force-computation means an output which is a function of the difference between such signal inputs, and switching means operable `during said time period to connect said sensed force signal to said input of said lock-on means and operable thereafter to connect said sensed force signal and said output of said lock-on means in opposition as said respective inputs to said signal-combining means to thereby render said control of said force-generator means a function of said difference.

3. The improvement as in claim 2 further comprising, additional time integrator lock-on means responsive to an input signal thereto to provide a held output representative of such signal as integrated over a time interval during which said material is rst passing through said preceding stand, additional signal combining means responsive to two respective signal inputs thereto to supply to said force-computation means an output which is a function of the difference between such two inputs, and

switching means operable during said interval to connect said additional signal to the input of said additional lockon means and operable thereafter to connect said additional signal and said output of such lock-on means in opposition as said respective inputs to said additional signal-combining means so as to render said. output from said signal-combining means a function of the difference between said additional signal and said output from said additional lock-on means.

4. In a mill control system in which material moves through successive stands and passes in each stand through adjustable working means to be reduced in thickness in accordance with the adjustment of such means, and in which force-computation means responds to a sensed force signal representative of force from said material on the working means of one of said stands to control forcegenerator means to apply to such working means a force which adjusts such means and is of a val-ue computed as being that required to produce a desired thickness reducing action on said material by said working means, the improvement comprising, time-integrator lock-on means responsive to an input signal thereto to provide a held output which is a function of such signal as integrated over a time period during which said material is first passed through said one stand, signal-combining means responsive to two respective signal inputs thereto to supply to said force computation means an output which is a function of the difference between such signal inputs to said signalcombining means, and switching means operable during said time period to connect said sensed force signal to said input of said lock-on means and operable thereafter to connect said sensed force signal and said output of said lock-on means in opposition as said respective signal inputs to said signal-combining means to thereby render said control of said force-'generator means a function of the difference between said sensed force signal and said output of said lock-on means.

5. In a mill control system in which material moves through successive stands and passes in each stand through adjustable working means to be reduced in thickness in accordance with the adjustment of such means, the improvement comprising, first and second load-sensing means of which each is associated with a respective one of two of said stands, each of said load-sensing means being responsive to the force from said materialon the working means of the corresponding stand to produce a sensed force signal which is a function of that force, first and second time-integrator lock-on means associated with, respectively, said first and second load sensing means and each responsive to an input to provide a held output which is representative of such input as integrated over a -time period of operation for that lock-on means during which said material is passing through the stand corresponding to that lock-on means, first and second signal-combining means associated with, respectively, said first and second lock-on means and each being responsive to two respective signal inputs from said lock-on means and said load sensing means to provide an output which is a function of the difference between such inputs, first switching means operable at the start of the integrating time period'for said first lock-on means to connect said output of said signalcombining means as said input to said first lock-on means, and operable after such time period to disconnect said output from said input of said lock-on means, second switching means operable at the start of the integrating time period for said second lock-on means to connect said output of said signal-combining means as said input to said second lock-on means, and operable after such time period to 1disconnect said output from said input of said lock-0n means and thickness control means responsive to the respective outputs of said two signal-combining means after each is receiving said respective signal inputs thereof to control the thickness reducing action of the working means of at least one of said two stands as a function of each of said two sensed force signals and each of the respective outputs of said two lock-on means.

1 0 References Cited UNITED STATES PATENTS CHARLES W. LANHAM, Primary Examiner.

10 A. RUDERMAN, Assistant Examiner.

Us. c1. Xn. 

